FS2K Specific Fuel Consumption Tutorial (by anonymous)

Posted by Sam Chin/FFDS
on Tuesday, 14 August 2001, at 9:01 p.m.

Well, I thought it was near impossible to solve, but an
"anonymous benefactor" emailed me a working solution to
modify FDE fuel consumption. (I have no idea if this was
published anywhere previously, but the author believes it is
unprecedented...please let me know if there is contrary evidence
to this). Note: it does not work for jets. In any case, I thank
again immensely the person who emailed this to me, and I reprint
the info here with permission:

-------------------BEGIN----------------------

FS2K Specific Fuel Consumption Tutorial.

This tutorial explains how to set SFC for piston engined and
turboprop powered aircraft in MSFS 2000.

The SFC of FS98 air files flown in FS2000 may be set using
this methodology, but it is not the preferred method.

This methodology should also work in CFS 1 and CFS 2 since the
piston engine code appears to be identical.

Setting SFC for engines without propellers is outside the
scope of this tutorial.

Until the publication of CFS1 specific fuel consumption for
aircraft with propellers was varied by setting data in section
500 of the air file. With the publication of CFS1 Microsoft began
to implement changes which allowed FDE to be varied from within
the aircraft.cfg instead of, or in place of, the filename.air.
Within FS2000 it seems to have been Microsoft's intention that
SFC for aircraft with propellers be varied from within
aircraft.cfg. To some extent it remained possible to set SFC for
piston engines, but probably not turboprops, by other more
complicated means. These are not discussed here.

Unfortunately there is no single entry for SFC in
aircraft.cfg. Setting SFC is however a simple process whether you
are authoring a new Flight Dynamics Envelope (FDE) for uploading
or just wish to alter the SFC of an aircraft which you have
downloaded.

With the exception of rotary and diesel engined aircraft the
SFC of piston engined aircraft differs only slightly at any
specified percentage of rated power. The aircraft. cfg
parameters;

[piston_engine] power_scalar = 1.0

[propeller] thrust_scalar = 1.0

are therefore frequently omitted by third party authors.

These are nevertheless the specific fuel consumption
parameters.

In principle to halve the fuel consumption without altering
the FDE you would instead include;

[piston_engine] power_scalar = 0.5

[propeller] thrust_scalar = 2

Half the power with double the thrust per horse power leaves
the thrust opposing the drag unchanged but cuts the fuel flow in
half. You will probably never need to make such gross changes in
practice.

To increase SFC by 10% you increase the power by 10% and
reduce the thrust to match.

[piston_engine] power_scalar = 1.1

[propeller] thrust_scalar = 0.909

Please note that you multiply and divide by 1.1. You must not
add and subtract 10%.

power_scalar * thrust_scalar must always =1

If they do not the FDE will alter and you have changed more
than SFC.

There are a couple of wrinkles however.

When you change mass fuel flow in unit time you will change
the energy flow and this will alter the engine temperature gauge
readings. This only matters if you are altering existing FDE. The
new readings will be more accurate, but if you wrote the original
FDE and you lovingly set the temperature gauges to read the
cruise numbers with the wrong SFC you will now have to go back
and fix those. Pressures will not alter.

The potentially larger wrinkle relates to the way that MSFS
handles any airscrew defined as constant speed.

When you alter the thrust scalar to fix bad SFC it will force
MSFS to change the prop pitch (slightly) to achieve the same
demanded rpm. It will then read a slightly different efficiency
from section 511. These changes are very small unless you make
gross changes in SFC. They may however put the FDE slightly out
of balance. Remember the new data are more nearly correct. Unless
you are very certain of the profile drag in the air file you
should change this very slightly as necessary to re- balance the
FDE.

If you have a test panel which will read thrust and you are
"fixing" a downloaded FDE and you are sure that it was
"accurate" apart from the SFC you may wish to take the
trouble to alter the thrust scalar in aircraft.cfg until the new
thrust exactly matches the old thrust at the speed which you
consider to be the most important for that aircraft. This is the
logical alternative to altering the profile drag. The thrust
before and after will not however match exactly at all speeds.

It follows that you should always research and write SFC data
to the aircraft.cfg before commencing testing of new FDE.

Although the first aircraft.cfg parameter is;

[piston_engine]

power_scalar =

It really means any engine with a prop. The piston engine data
in the aircraft.cfg is read if the engine is defined as turboprop
in the air file. It follows that the procedure for setting the
SFC of turboprops is identical. You may need to make larger
variations and the need to re- balance FDE for c/s prop pitch
changes becomes more likely.

The only wrinkle is that some FDE authors may set one or other
of the aircraft.cfg parameters for a turboprop aircraft to a
value other than 1.0 for reasons not related to SFC. Proceed as
before but multiply and divide pre SFC factor parameters by the
difference between default SFC and prototype SFC.

When altering existing turboprop FDE calculate power_scalar *
thrust_scalar. The answer after the SFC change must be the same
as before. For a turboprop it need not be exactly 1.

Three caveats;

1) If you make gross changes to the scalars it is possible
that in certain regimes of flight you may force MSFS to try to
read beyond the end of the prop efficiency tables. In these
circumstances it may not be able to calculate an appropriate
airscrew efficiency and the FDE will alter. If you understand
sections 510-512 of the air file you may be able to rewrite the
data to resolve this. Unless you are prepared to undertake this
task it is necessary to recognise that there are undefined
boundaries beyond which SFC cannot deviate from the default
without imposing errors in the thrust equation. These boundaries
vary from aircraft to aircraft depending to a large extent, but
not solely, on the prop data in sections 510-512 of the air file.
If you stick to real world SFCs you will probably never encounter
the boundaries.

2) Using the scalars really does alter the horse power data
output. Remember this when using a test panel prior to FDE
upload. The required reading on the test panel is real world
horse power * power scalar. The thrust gauge on the test panel
should change only slightly whatever the SFC, (see above re c/s
props in FS2000).

3) I believe there have been real world aircraft with
(equivalent / shaft) horse power gauges on their panel but AFAIK
these do not exist in MSFS except as test gauges. So long as MSFS
(prop) panels have only gauges to enable power management by
boost, rpm, or fuel flow the user will not be aware that the
program is outputting scaled horse power which are rescaled to
give the correct thrust. In the end it is always thrust not power
which opposes drag.

Aircraft in which power management is by reference to fuel
flow will of course benefit most from application of this
tutorial.

Finally I recommend that you explain your aircraft.cfg entries
to users so that the aircraft.cfg might read;